subhaanallah

Q:

1. What is the minimum duration for sujood, qaumah and ruku without which salah will be invalid?

2. If one says Subhana Rabbiyal Azeem and Subhana Rabbiyal Aala once only due to time constraints and if leading salah says Sami Allahu liman Hamidah and goes to sujood, would it suffice for salah to be valid or one has to wait for muqtadees to say Rabbana lakal Hamd?

3. If while leading salah, and in ruku the imam feels by the sounds of steps or running to catch the rakat, is it permisible for the imam to prolong ruku to allow them to catch the rakat?

Q: What is the ruling if a person lifts up both his legs during sajdah? Is the prayer invalidated?

Q: If a person is praying Zuhr salah at home and a family member calls him from another room, what is the best way to notify the family member that he is in Salaah? Is it ok for him to recite his qira'ah loudly for 1-2 seconds to show that he is praying (even if it is Zuhr salat), or should he say "subhaanallah" loudly or is there any better way?